The food industry has recently focused considerable attention on the production of polyol fatty acid polyesters for use as low calorie fats in food products. As a result, there is a continuing need for processes which economically and efficiently produce a relatively high purity polyol fatty acid polyester.
Certain nondigestable polyol fatty acid polyesters have been found to be useful as low calorie substitutes for triglyceride oils. For example, Mattson et al., (U.S. Pat. No. 3,600,186, issued Aug. 17, 1971), discloses low calorie cooking and frying oils in which at least a part of the triglyceride oil is replaced by a nonabsorbable, nondigestable sugar fatty acid ester or sugar alcohol fatty acid ester having at least 4 fatty acid ester groups with each fatty acid having from 8 to 22 carbon atoms. Bernhardt, (European Patent Application 236,288, Published Sep. 9, 1987), and Bernhardt, (European Patent Application 233,856, Published Aug. 26, 1987), disclose certain intermediate melting polyol polyesters which can be used as replacements for at least a portion of the triglyceride oil in cooking and frying oils and which provide passive oil loss control. Blends of completely liquid polyol polyesters with completely solid polyol polyester hardstocks, preferably esterified with C.sub.10 to C.sub.22 saturated fatty acids (e.g., sucrose octastearate or octabehenate) have also been proposed in order to provide passive oil loss control. See, for example, Jandacek; U.S. Pat. No. 4,005,195 and Jandacek/Mattson, U.S. Pat. No. 4,005,196; both issued Jan. 25, 1977.
Another type of nondigestable polyol polyester composition which can be used to replace part or all of the triglyceride oil in cooking or frying oil comprises combinations of liquid polyol polyesters and certain types of solid particulate material selected so that the composition has an essentially flat Solid Fat (SFC) profile slope over the temperature range between room temperature and body temperature. Generally, the solid material in such composition is present as very small particles (1 micron or less) and at relatively low concentration. Frequently, such solid particulate material, which serves as a passive oil loss control agent, will be a solid polyol polyester that crystallizes into the desired especially small particles. Examples of polyol polyester compositions of this type, and of cooking and frying oils containing them, are described in Young, U.S. Pat. No. 5,085,884, issued Feb. 4, 1992; Letton et al, U.S. Pat. No. 5,306,514, issued Apr. 26, 1994, and Letton, et al, U.S. Pat. No. 5,422,131, issued Jun. 1, 1995; and U.S. Pat. No. 5,534,284 to Corrigan et al., issued Jul. 9, 1996.
To produce a polyol fatty acid polyester, a polyol can be reacted with a fatty acid lower alkyl ester in the presence of a basic catalyst. In general, polyols are readily soluble in an aqueous medium, e.g., water, while fatty acid lower alkyl esters are soluble in an organic medium. Thus, an emulsifier, solvent, phase transfer catalyst or a mixture thereof is usually required to bring the polyol and the fatty acid lower alkyl ester into physical contact so that they can react chemically. The resulting polyol fatty acid polyester is soluble in an organic medium.
Nondigestable polyol polyesters are typically prepared by a solvent-free, essentially two-step transesterification of the polyol (e.g., sucrose) with the fatty acid esters of an easily removable alcohol (e.g., fatty acid methyl esters). In the first step, a mixture of sucrose, methyl esters, alkali metal fatty acid soap and a basic esterification catalyst are heated to form a melt. In the second step, an excess of methyl esters are added to this melt which is then heated to convert the partial sucrose esters to more highly esterified sucrose polyester. See, for example, Rizzi et al, U.S. Pat. No. 3,963,699, issued Jun. 15, 1976; and Volpenheim, U.S. Pat. No. 4,517,360, issued May 21, 1985.
Alternatively, highly esterified polyol polyesters can be prepared by two stage solvent-based processes (see, for example, Masaoka et al, U.S. Pat. No. 4,954,621, issued Sep. 4, 1990) or one stage solvent-based or solvent-free processes (see, for example: Van der Plank U.S. Pat. No. 4,968,791, issued Nov. 6, 1990; Meszaros Grechke et al, U.S. Pat. No. 5,079,355, issued Jan. 7, 1992; or Van der Plank et al, U.S. Pat. No. 5,071,975, issued Dec. 10, 1991).
As can be appreciated, the product stream resulting from the reaction of a polyol to produce a polyol fatty acid polyester can therefore contain a variety of components in addition to the desired polyol fatty acid polyester. For example, residual reactants, e.g., unreacted fatty acid lower alkyl ester and/or unreacted polyol, emulsifier, solvent, phase transfer catalyst and/or basic catalyst can be present in the product stream. Additionally, there can be numerous by-products of the reaction itself. For example, numerous side reactions occur in addition to the transesterification of the polyol to form a polyol fatty acid polyester. Side reactions can include the breakdown of one chemical component into two or more by-products, and/or the initial reactants, catalysts, emulsifiers and solvents can chemically react with one another to form undesired by-products, for example, di- and tri-glycerides, beta-ketoesters, di-fatty ketones, and saturated and unsaturated fatty acids and/or soaps. These unsaturated and saturated fatty acids and soaps can result from the hydrolysis of the polyol polyester, and of the starting methyl esters. Additionally, the initial reactants and other reaction ingredients are often supplied with trace quantities of materials, e.g. trace metals, including calcium and magnesium ions, which are particularly undesirable in a final product which is intended for use as a food additive. Thus, the product stream resulting from the reaction of a polyol and a fatty acid lower alkyl ester can contain, in addition to the desired polyol fatty acid polyester, a variety of undesirable constituents as impurities which need to be substantially removed to yield the desired purified polyol fatty acid polyester. These impurities can contribute to instability and/or discoloration of the polyol polyester, especially during cooking or frying. In general, it is therefore necessary to further refine or purify the crude polyol fatty acid polyester reaction products resulting from such conventional synthesis.
Conventional purification methods include washing with water, extraction with organic solvents and/or "salting-out" treatments. U.S. Pat. No. 4,334,061 describes sucrose polyesters preparation in which the reaction product is washed using an aqueous alkaline solution of pH 7-12 in the presence of a polar organic solvent. Van Lookeren, U.S. Pat. No. 5,055,571, issued Oct. 8, 1991, discloses a process for the purification of crude polyol polyesters by contacting the polyol polyesters with alkali metal ions under alkaline conditions to reduce the level of alkali metal ions to less than 5 ppm calculated by weight of the polyol polyesters. In this process the monovalent soap level present in the crude polyol polyester is preferably reduced by centrifuging or "filtering off" crystallized soap and/or one or more washings with water at near neutral conditions.
Watanabe, U.S. Pat. No. 4,973,681, issued Nov. 27, 1990, describes a purification process for increasing the oxidative stability of a polyol polyester comprising contacting a polyol polyester with a polybasic oxy-acid (e.g., citric acid) and then separating the polyol polyester from the polybasic acid. The reference discloses that this process removes trace amounts of metal catalyst which may be present in the crude polyol polyester.
Even when nondigestable polyol polyesters are purified as described above, the are not as stable against hydrolysis during frying as conventional triglyceride oils. Therefore, cooking and frying oils that comprise these nondigestable polyol polyesters develop an off-flavor faster than triglyceride frying oils and often discolor more quickly. It would, therefore, be desirable to prepare nondigestable polyol polyester cooking and frying oils which are equivalent to triglyceride oil in terms of hydrolytic stability and color development. The more stable the polyol polyester composition, the longer the fry-life of these oils.
It has been discovered that, in addition to the monovalent soaps discussed above, polyol polyesters can contain a significant amount of divalent metal ion higher soaps (e.g., calcium soap), and further that these divalent and higher-valent metal soaps have a deleterious effect on the hydrolytic stability of the polyol polyesters when, in addition, there are free fatty acids are present. These divalent and higher-valent metal ions are usually introduced into the polyol polyester as contaminants of the starting methyl esters. Free fatty acids can come into the polyol polyester from the food being fried, or by hydrolysis of the polyol polyesters, or both.
Therefore, a continuing need exists to improve the separation and purification of a polyol fatty acid polyester reaction product stream, particularly resulting from the transesterification of a polyol. More specifically, it is desirable to provide an economical and efficient separation process which can remove divalent and higher-valent soaps, and trace metals such as calcium and magnesium, as well as fatty acid methyl esters and fatty acids.